The present invention relates to electronic circuits and, more particularly, to a phase-controlled power modulation system. A major objective of the present invention is to provide for improved power control during a circuit-threatening condition of a phase-controlled (PC) power system, for example, of the type used as dimmers in lighting installations.
The ability to modulate the electrical power delivered to a load has been a requirement in both commercial and industrial applications. In particular, lamp dimmers in the commercial and entertainment lighting fields have been driven by the requirements of lighting designers for flexible and reliable control, along with the public demand for sophisticated special effects. These forces have motivated the development of modern phase control solutions for the control of incandescent lighting.
Phase controlled systems modulate delivered power by decoupling a load from an alternating current (AC) source for a fraction of each half cycle. Longer decoupling lowers delivered power, while shorter decoupling increases delivered power. Maximum power is delivered when there is no decoupling. Forward phase control (FPC) systems modulate power by decoupling at the beginning of each half cycle. Of the power associated with a half-cycle, only that fraction associated with the portion of the power curve occurring after coupling is delivered to the load. On the other hand, reverse phase control (RPC) systems begin each cycle with the load coupled to the power source. A power level below maximum is achieved by decoupling at the appropriate phase before the end of the half cycle. Both FPC and RPC systems have been implemented, although RPC is presently favored because it is less subject to noise generated by incandescent lamps due to mechanical resonance, commonly known as "lamp sing".
Phase control is typically implemented using a switch assembly with two complementary switches. Earlier FPC systems use an inverse parallel pair of silicon controlled rectifiers (SCRs). A magnetic choke is required in series with the SCRs to limit the rate of rise of the voltage at turn-on to avoid electromagnetic interference (EMI) and radio frequency interference (RFI), and to reduce noise. For example, a choke would establish a minimum transition time of 400 .mu.sec. Magnetic chokes are relatively heavy and expensive and thus undesirable, especially in applications such as theater lighting that require independent dimming of many lights.
More recently, phase control has been achieved by switch assemblies having a pair of voltage-controlled switches (VCSs) in inverse series relationship. Typically, VCSs can be insulated-gate bipolar transistors (IGBTs) or metal-oxide silicon field-effect transistors (MOSFETs). The transition time of these VCSs can be controlled as these switches pass through their active region to mimic the action of the choke used with SCR switches. For example, the transition through the active region of a VCS can be set to about 400 .mu.sec to minimize interference and noise. Accordingly, chokeless and thus more economical and compact dimmer systems have been achieved.
Power control systems are subject to a variety of conditions since a variety of power sources and loads can be coupled through the switches. VCSs, even more so than their predecessors, are subject to damage or performance impairment by a variety of device-threatening conditions. These include excessive currents, voltages and temperatures.
Accordingly, practical power control systems using VCSs include protective circuits that handle these threatening conditions. In some cases, these circuits shut down operation until the condition is removed. Preferably, however, operation is altered in some way to permit the power control system to continue functioning, albeit with tradeoffs.
An example of a device-threatening condition is an excessive current (overcurrent) caused, for example, by a short circuit or a large load. Such an excessive current can destroy the VCSs. Therefore, some overcurrent protection, generally a current limiter, is provided in a VCS power control system. A tradeoff of current limiting is noise generated with the sharp waveform transitions caused by limiting.
A complex overcurrent situation occurs when bringing an incandescent lamp from a non-incandescent state to an incandescent state. The cold resistance of an incandescent lamp filament typically increases by an order of magnitude as the lamp incandesces and the filament heats. As a result of this phenomenon, surge currents of over 10 times the steady state requirements are encountered when a non-incandescing incandescent lamp is energized. In prior art SCR/choke dimmers, these surge currents are tolerated through the use of the choke to reduce their magnitude, and by overspecifying the SCRs and circuit protection devices (circuit breakers or fuses) used for a particular application, i.e. 20 Amp dimmers typically require 40 Amp SCRs. This solution is expensive, and also shortens the life of the bulb as associated tremendous surge currents cause physical stress to the lamp filament.
One might presume, therefore, that in a dimmer employing VCSs, this problem can be solved simply by setting the current limit for the dimmer to that required for lamp incandescence, i.e. 10 times the steady state value. Unfortunately, this is impractical, +VCSs able to operate at these current levels would be prohibitively expensive and result in a non-competitive product. However, setting the current limit only at the relatively low steady state current draw of the lamp will not allow the dimmer to deliver sufficient energy to a cold lamp to cause it to incandesce in an acceptable time period. Clearly, a mechanism that will allow a low current limit setting, yet ensure timely cold lamp response, is required.
Voltage limiting is also required to avoid damage and performance impairment by excessive voltages. Certain load types, namely inductive loads operating on an RPC dimmer, can induce voltage surges that require voltage limiting. Known voltage limiting devices limit voltage by converting excess voltage to heat, which must then be dissipated. The conversion of voltage to heat represents a loss of efficiency. Furthermore, the heat sinking required to dissipate the heat can add considerably to the bulk and expense of a power control unit. Finally, the heat so generated can contribute to a threat of thermal damage to the circuit, discussed immediately below.
Excessive heat can damage and impair the performance of a phase-controlled switching system. Where the switching system is colocated with the luminaire it is controlling, its ambient temperature can be very high since over 95% of the energy of a typical incandescent lamp is dissipated as heat. In addition, the nature of theatrical and architectural lighting typically requires that the luminaire be located at some height above the floor, therefore being located in a thermally hot portion of an enclosed room. Finally, installation errors can block the convection currents necessary to allow for cooling of a non-forced air cooled dimmer.
Whether or not the switching system is colocated with its luminaire, heat is generated by the switch assembly due both to conductive losses and to transition losses. As noted above, heat can also be generated by limiting devices.
As previously described, lamp sing and EMI/RFI are controlled by maintaining the VCS in its active region for the transition period. Since the VCS is actually functioning as a voltage-controlled resistor during this time, the VCS semiconductor junction is subjected to a very large thermal pulse. This thermal pulse is a function of the current through, voltage across, and the transition time of the VCS. During normal operation, the VCS components can be sized to handle the worst case thermal pulse. However, during overcurrent limits as described above that would be required to allow a cold lamp to incandesce, the increased current can cause excessive thermal pulses. U.S. Pat. Nos. 4,633,161 and 4,823,069, both to Callahan et al., claim that the solution to this problem is to reduce the transition time in response to an overcurrent situation. This is an unacceptable solution since it results in increased lamp noise during warmup, a period when the lamp is naturally noisier than normal due to the thermal and mechanical stresses that occur as the filament incandesces.
Dissipation of the heat so generated can be degraded by excessive ambient temperature, impeded air flow, or incorrect installation. The resulting heat accumulation can damage the switch assembly. Thermal protection can be implemented by sensing temperature and shutting down a system before damage occurs. The obvious tradeoff is that shutdown can be very inconvenient.
Electrical noise can also be device threatening. Electrical noise can be generated by sharp transitions in electrical voltage. This noise can affect the microcontroller of a microcontroller-based dimmer system, possibly causing erroneous actions. For example, noise can mask an indication requiring a shutdown; the resulting failure to shut down, e.g., in response to an overcurrent or overvoltage, can result in damage to the switches. Prior art attempts to limit electrical noise have involved controlling the transition rate of the switches. Nonetheless, the remaining noise can be unacceptable.
Thus, the prior art provides a variety of schemes for addressing device and performance-threatening conditions. However, these schemes generally involve undesirable tradeoffs. What is needed is a power control system that provides the necessary protective measures while minimizing performance impairment when these measures are functioning.